First Super Sabre ZLL

Fifty-nine years ago this month, the first zero-length launch of an USAF/North American F-100D Super Sabre took place at Edwards Air Force Base, California. North American Test Pilot Albert “Al” R. Blackburn was at the controls of the experimental rocket-propelled test aircraft.

Zero-Length Launch (ZLL) was a concept pioneered by the United States Air Force for quickly getting aircraft into the air without the need for a conventional runway take-off. The idea was to launch the aircraft via rocket power from a standing start. Shortly after launch, the aircraft was at flight speed and the dead rocket booster was jettisoned.

The ZLL concept was typical of attempts by USAF to gain tactical advantage over the Soviet Union in any way possible during the Cold War era. ZLL held the promise of getting nuclear-armed combat aircraft airborne in the event that airfield runways were rendered useless due to enemy attack.

The ZLL mission was envisioned as one-way trip. Once airborne, a pilot was expected to fly to the target, deliver his nuclear weapon and then fend for himself. Survival options included finding a friendly field to land at or eject over non-hostile territory. Either way, the pilot’s chances of things working out well were not good.

The first attempt to test the ZLL concept dates back to 1953 when USAF employed the straight-wing USAF/Republic F-84G Thunderjet in the ZELMAL Program. ZELMAL stood for Zero Length Launch/ Mat Landing. This last part involved the aircraft making a wheels-up landing on an inflatable rubber mat that measured 800 ft x 80 ft x 3 ft.

Piloted ZELMEL flight tests were conducted at Edwards Air Force Base in 1954. The results were not particularly encouraging. Understandably, the mat landing was a rough go. The landing mat was heavily damaged on the first test and the pilot sustained back injuries that required months of recovery time. Subsequent flight tests showed ZELMAL to be essentially unworkable as then conceived.

USAF reprised the ZLL concept in 1958 with the F-100D Super Sabre. Gone was the mat landing part of the operation. The aircraft would make a normal wheels down landing if it got airborne successfully. Launch motive power was provided by a single Rocketdyne XM-34 solid rocket booster that generated 132,000 lbs of thrust for 4 seconds. At burnout of the boost motor, the F-100D was 400 feet above ground level and traveling at 275 mph. The booster was then jettisoned and the aircraft continued to accelerate under turbojet power.

The first flight test of the F-100 ZLL aircraft (S/N 56-2904) took place on Wednesday, 26 March 1958. The aircraft was launched from the south end of Rogers Dry Lake at Edwards Air Force Base, California. The test aircraft carried a pylon-mounted 275-gallon fuel tank on the right wing and a simulated nuclear store on a pylon under the left wing. North American Aviation’s Albert “Al” R. Blackburn was the assigned test pilot.

The first F-100 ZLL flight was entirely successful. Blackburn reported an exceptionally smooth ride on the rocket and an uneventful transition to turbojet-powered flight. While the boost motor struck the belly of the aircraft during departure and caused some superficial damage, the separation was otherwise successful. After a brief flight, Blackburn safely landed his steed on the concrete.

The first flight success of the ZLL Super Sabre was followed several weeks later by loss of aircraft 56-2904. Although the launch phase was proper, the boost motor hung-up on its support bolts and could not be jettisoned following burnout. Blackburn’s attempts to shake the dead motor free from the aircraft proved futile. Since a successful landing was not possible, the pilot was forced to eject.

A total of fourteen (14) more F-100D ZLL flights took place at Edwards through October of 1958. Although these flights proved ZLL to be a viable means for getting a combat aircraft airborne without the use of a conventional runway take-off, USAF ultimately judged the concept to be too impractical and logistically burdensome to field. This in spite of the fact that a total of 148 F-100D Super Sabre aircraft were eventually modified for the ZLL mission.